462 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 27, 
that at the bottom a portion of the sinus is isolated. The uncus 
arises from the posterior column at a very acute angle ; but owing 
to its rapidly making a curve outwards, its general direction is 
nearly parallel with that of the column; and the short reentering 
angle is occupied by a slender columella, d, rising from the bottom 
of the sinus. A short crista (“ anterior combing-plate’’), e, projects 
into the sinus on its outer side, and in a line nearly vertical to 
the uncus, with which, however, it is in no way connected. What 
remains of the dorsum or “ outer lamina” shows no elevation of 
the first or angular costa (f), but a considerable elevation of the 
second (g); the remainder of the surface is evenly undulated, with- 
out any distinct indication of a fourth costa. The anterior column 
is widely expanded at its inner end, with a rather deep sinuosity 
on its anterior aspect (A). The enamel throughout is very thin, and 
where the surface is exposed it 1s smooth. Towards the base of 
the dorsum there is a thin irregular coating of cementum. 
Tn the opposite tooth, no. 877 (Fig. 2), we are fortunately furnished 
with exactly the parts that are wanting in no. 878, viz. the posterior 
column, &c. The tooth is so much worn that the posterior sinus 
(“ valley ”’) is represented merely by a small circular pit (2): itis con- 
sequently impossible to ascertain whether the hinder vallum (“third 
collis”) was cuspidate or emarginate. In all other respects the tooth 
presents exactly the same characters as its opposite fellow, except 
that the entrance of the median sinus is furnished with only a single 
cusp, and that, owing to its being rather more worn, the uncus 
extends quite across the sinus, and becomes confluent with the an- 
terior column, so as completely to isolate the external portion of the 
sinus, and produce a third pit or fossette. But it will be observed 
that this fossette is not formed in the same way as the peculiar 
‘“‘tichorhine pit” (that 1s, by the coalition, ab initio, of the uncus 
and erista), but simply, as not unfrequently occurs, by the prolon- 
gation at the base of the former, so that it reaches as far as the 
anterior column—a circumstance obviously of little moment as a 
character, since, as we here see, it may be said to exist on one 
side and not on the other. It may also be added that a similar 
prolongation of the uncus has been previously noticed in teeth of 
RR. leptorhinus, an instance of which is afforded in pl. 51. fig. 4 of 
the ‘ Ossemens Fossiles,’ which plainly represents, as it seems to me, 
a tooth of FR. leptorhinus, and not of R. hemitechus as supposed by 
Prof. Owen. And a similar instance is shown in the tooth figured 
by Mr. W. B. Dawkins (J. ¢. p. 410, fig. 10), I believe, from Grays 
Thurrock. And it is important to remark the occasional possible 
occurrence of this peculiarity, smce M. Christol, in his definition of 
R. megarhinus, expressly says, “le crochet de leur colline pos- 
térieure ne se joint jamais 4 l’antérieure ” *. 
* Dr. Falconer also (op. cit. ii. p. 336, pl. xviii. fig. 5) notices and figures a 
similar occurrence in a molar of R. hemitechus, and remarks:—‘ That this 
peculiar confluence of the crochet with the anterior barrel is abnormal in the 
true molars, is proved by the extreme rarity of the instances which have been 
observed of it in any species of Rhinoceros.” He then cites Cuvier’s figure, to 
